Technical Field
The present invention relates to an electrophotographic image forming apparatus, such as a copier, a printer, or a facsimile machine.
Description of the Related Art
In an electrophotographic image forming apparatus, an electrostatic latent image is first formed in an image forming section, the electrostatic latent image is developed by toner as a developer and is rendered visible as a toner image, the developed toner image is transferred to a recording sheet by a transfer device, and subsequently, the image is fixed onto the recording sheet as a final image by a fixing device.
The fixing device includes a heating member and a pressure roller that press against each other to form a fixing nip, through which the recording sheet on which an unfixed toner image is borne passes. The recording sheet is pressed and heated when passing through the fixing nip (i.e., between the heating member and the pressure roller), so that the toner image is fixed onto the recording sheet.
In this type of heat fixation, there is a difference in temperature between the heating member and the pressure roller, and therefore between the front and back side back side of the recording sheet. Specifically, the surface of the recording sheet contacting the heating member shows a higher temperature than the back side of the recording sheet contacting the pressure roller.
As a result, after passing through the fixing nip, moisture contained in the recording sheet evaporates more from the front surface of the recording sheet than from the back side, and the moisture moves from the back side to the front side. As a result, moisture inside the front side becomes greater than that in the back side and extension of fibers in the front side becomes greater, so that the recording sheet curls to the back side (called a back curl).
Recently, in particular, the heating member is formed to have a lower thermal capacity for a quick rise in the temperature to save energy and shorten warm-up time. With such a fixing device, because feeding sheet becomes ready before the heating member is satisfactorily warmed, a difference in the temperature between the heating member and the pressure roller increases and the difference in the temperature between the front and back side of the recording sheet in the fixing process tends to be greater. Thus, a back curl occurring in the fixing nip becomes pronounced. In such a fixing device, when the recording sheet is ejected with a large back curl, the rated number of recording sheets cannot be stacked on a sheet tray, or alternatively, the stacked sheets in the tray are messed up.